Fluidics was a competing technology to solid-state electronics in the 1960's and 1970's [Belsterling, Charles A., Fluidic System Design, 1971, Wiley Interscience; Conway, Arthur, A Guide to Fluidics, 1972, MacDonald and Co.]. Device physics for these fluidic devices was based primarily on inertial effects in fluid-like jet interaction, working on the basis of inertial forces present at larger (˜1 cm) scales (higher reynolds number). Several large-scale all-fluidic control systems were demonstrated during that time. Because viscous and surface tension forces dominate fluid dynamics at small scales, these devices could not be miniaturized further, resulting in limitations in large-scale integration. Fluidic approaches to control and logic applications were therefore eventually abandoned due to the inherent disadvantage that they could not be scaled down below millimeter scale because of their dependence on inertial effects. Furthermore, fluidic technology in the 1960's primarily used analog representations. This did not provide the state restoration benefits obtained with digital logic.
Various researchers have tried to exactly scale down the inertial effect devices using silicon micromachining [Zemel, Jay N., “Behaviour of microfluidic amplifiers, Sensors and Actuators, 1996]. As expected, the performance of these inertial effect devices falls down sharply with smaller length scales. High pressure and fluid flow velocity can be employed to improve upon performance, but this approach is not feasible if good performance for fluidic devices is required at reasonable pressure differentials.
Scalable control of droplet based microfluidic systems is one route to integrated mass-processing units at miniature length scales. Currently used external electronic control schemes use large arrays of electrodes, such as in electrowetting-based microfluidic droplet systems, thus limiting scaling properties of the devices. Moreover, electric fields can cause unwanted interference effects on biomolecules. The problem is further complicated by difficulties arising due to packaging and merging of silicon based technology with PDMS based soft lithography techniques. Due to the absence of a scalable control strategy for droplet based microfluidic systems, most droplet systems are currently designed as linear channels. Multi-layer soft lithography-based microfluidic devices use external solenoids that are much larger than the fluidic chip and are external to the device. As the complexity of the chip increases, the number of control lines increases drastically, making it intractable as a scalable control strategy. Moreover, control elements made using multi-layer soft lithography cannot be cascaded, resulting in limitation of scaling. As an analogy to the microelectronics revolution that occurred in the 1960's and 1970's, massive scaling of electronic circuits was only possible by moving every element of the circuit on a single integrated chip itself. Similarly, for micro-fluidic chips to provide the same complexity commonly seen in electronic counter parts, all control and logic elements must be designed to be completely on-chip.
Table 1 lists relevant forces in fluid dynamics and their dependence on Reynolds number, with examples of their use as a flow control technique.
TABLE 1ReProgrammabilityFlow control eg.* Surfaceindependentsurface energyPassiveTensionpatterning; D.capillary valvesBebee et al.and controlBoundary layerRe > O(100)Structure of theDrag reductionseparationchannelusing active controlElectro-hydroRe < O(10)High V electrodesElectro kineaticdynamicintegrated inchipsinstabilitiesmicrochannels* Two phaseindependentdevice structureNoneflowInertialhigh; Re >flow interactionDiodes, triodes,forcesO(500)amplifiers, gatescentrifugal force“lab on CD”WallRe > O(100)flow interactionbistable amplifiersattachment
An all-fluid control and logic circuit using non-newtonian fluids was proposed recently [Groisman, Alex et al., “A microfluidic rectifier: Anisotropic flow resistance at low Reynolds numbers”, Physics Review Letters, 2004; Groisman et al., “Microfluidic memory and control devices, Science, 2003]. Several devices, including a bistable memory and a microfluidic rectifier, were proposed. The nonlinearity of the system comes from using non-newtonian fluids. A polymer-based solution is used as the acting fluid, with polymer chains stretching and compressing, which provides a nonlinear behavior to the fluid. Use of non-newtonian fluids severely limits the applicability of these devices in various situations.
Various control strategies for microfluidic devices have been proposed using thermally generated vapor bubbles. Thermally generated bubbles from micro-heating elements have been previously used in ink-jet applications. A vapor bubble is used to push on a fluid layer that is ejected out of the channel. A mechanical structure can also be moved using a thermally generated vapor bubble [Schabmueller, C G J et al., “Design and fabrication of a microfluidic circuitboard”, Journal of Micromechanics and Microengineering, 1999]. However, the device requires integration of heating elements in fluidic channels with mechanical structures, and the control is limited by the rate of generation of thermally induced vapor bubbles. Thermally generated vapor bubbles are transient in nature, and vapor bubbles dissolve in surrounding liquid as soon as the heat source is removed, so any effect caused by presence of vapor bubbles is short lived. Using a heating element for bubble generation also results in unwanted thermal effects on the biomolecules and reactions being carried in the microfluidic device.
Previous fluid logic demonstrations at low reynolds number therefore have various shortcomings, including use of non-newtonian fluids, with consequent non-linear flow properties, use of an external switching element like a solenoid, limiting achievable device speed, difference in representation of input and output signal thus inability to cascade logic gates to form a complex boolean gate, and an inability to scale to large and complex microfluidic droplet/bubble circuits. In addition, there is a limitation in providing input to microfluidic chips, because the input must be provided serially using valves based on solenoids located outside the chip. With increasing complexity of the chips, more and more information needs to be input into the system, so this limitation results in a bottleneck. In addition, the number of control lines needed to run a microfluidic chip currently increases drastically with the complexity of the designed chip. This is because the switching elements cannot be cascaded to form complex control networks. What has been needed, therefore, is a system that uses only newtonian liquids, logic elements that are cascadable, exhibit gain and fan-put, and can switch faster than previous devices, and a system that is scalable to large and complex microfluidic droplet/bubble circuits.